Damping Factor: Soggy at Best

Of all the misunderstood specifications in this industry, Damping Factor (DF) probably receives more misplaced significance than anything. While, aside from a few exceptions, manufacturer spec sheets are broad generalizations that shouldn’t be used by anyone as the primary basis to compare anything, the Damping Factor spec is especially meaningless when used to compare or classify amplifiers. It’s overblown for one very simple reason: cabling and loudspeaker resistance has a far larger effect on actual damping factor than the amplifier can ever hope to.

To illustrate my point. consider the QSC PL340, an amplifier with a damping factor of 500, which is relatively standard for a pro amp. To take this out of the spec sheet and into the real world, let’s say I attach an 8Ω loudspeaker to that amplifier with 50′ of 12 gauge cable. We’re not just talking about the amplifier now, we’re dealing with a system comprised of various resistances, including the loudspeaker and cabling. With a little bit of calculation, I find that my system’s damping factor is now 49.14… good enough for even recording studio situations (you want about 20 for PA use, more like 50 for critical listening).

That’s all well and good, but of course a bigger number is better, so how about running the numbers again, this time with the Crown I-Tech 8000? This amplifier has a stated damping factor of 5000, an order of magnitude greater than the PL340. Redoing all my painstaking calculations, I find that attached to the same 8 ohm loudspeaker with the same 50′ of cable the result is a damping factor of 50.25, not even 1% better!

To really explore the relative insignificance of an amplifier’s inherent damping factor, imagine we could have an amp with a damping factor of 500,000. What would that amplifier’s DF be after it is plugged into our imaginary speaker system? A measly 50.38, it turns out, an insignificant difference even though this amplifier has 100 times the damping factor of the I-Tech 8000 and 1000 times the damping factor of the PL340.

OK, let’s examine the other extreme, what about an amplifier with an abnormally low damping factor, say 100? The end result is still a remarkably good net DF of 44.74, not even 12% worse than our imaginary half-million DF amp.

That’s not to say damping factor isn’t important, quite the opposite, it should be kept as high as possible to make sure your PA sounds as good as it can. All I’m saying is that the amplifier itself has almost nothing to do with the system’s damping factor equation, which is largely controlled by the resistance of the loudspeakers and cabling attached to that amplifier.

Just how much do the cabling and loudspeakers affect that important overall damping factor spec? To find out, instead of leaving the speaker and cabling the same and switching amplifiers, I’m going to show what happens when you go from the “perfect” world of 8 ohms per channel and 50′ of 12 gauge cable to the real world, where that almost never happens.

What a dramatic difference, and all I did was run a few more speakers (or lower ohm drivers) per side, or double the length of cable to my speakers. 100′ of cabling isn’t at all unusual, and look at how badly it destroys your overall DF if you load it down to 2Ω… 100′ of 12 guage cabling with even an 8 ohm speaker, an easy load to be sure, is just over the DF you want for quality PA use.

Now that you hopefully have a basic understanding of what damping factor is and how it manifests in real systems, you’re no doubt interested in the best way to keep it at a reasonable level in your sound systems. The answer? Cable diameter. As a fairly real world example, I’m going to assume 100’ of cabling and a 4 ohm load, which is a pretty good balance between overloading or under-loading your amps. Those of you running your systems at 2Ω per channel, especially in the very low frequencies, stop it. Here’s what the damping factor of that system looks like with various gauges of cabling:

As you can see, while damping factor is an important thing to keep in mind, cable thickness is by far the dominant factor. Unfortunately, it’s difficult to get better than 10 AWG into a Neutrik Speakon (and don’t even talk to me about banana or, worst of all, 1/4″!). Long story short: use 12 or 10 AWG cabling, keep your loads 4 ohms or higher, and your cables 100′ or shorter!

A parting shot: Let’s say you had 2′ of 16 AWG cable between an amplifier with a DF of 500 and a 4Ω speaker… your net DF would be 199.36! Of course, the only way to achieve this is to integrate the amplifier(s) into the loudspeaker enclosure, and who wouldn’t want to do that?

Damping Factor Defined

Scientifically speaking, Damping Factor is the ratio of the resistance component of the loudspeaker’s impedence to the resistance component of the amplifier’s output impedence. This number is used to determine how well the amplifier is able to control the motion of the loudspeaker. Too low and the amplifier won’t be able to resist back-EMF from the speaker and the cone will “flop around” a little, resulting in a less clear reproduction of the signal being sent to it.

This specification is almost always given with a perfect 8Ω load attached to the amplifier, operating at a fairly low frequency (<1kHz).

3 COMMENTS

[QUOTE=;][/QUOTE]
Sorry Bennett – I’m guilty of running subs at 2 ohm – Xti 6000 8×18’s – I know it’s not good but that’s the way it is for now. On the plus side – I did make a bunch of 10 gauge cables. I need to make a few more but am dreading the chore – it’s really tuff to squeeze all that into those newfangled Neutriks. Your article confirms my ideas about placing amp racks on either side and using shorter fatter cables. When I grow up that’s just what I’ll do.

Aside from the damping factor issues, which are frankly kind of nit picky “last 5%” kind of optimizations, my concern with loading an amp down to 2 ohms a side is: what happens if it fails? I’m not saying that running it at maximum load will cause it to fail in and of itself, although for sure the extra thermal load is not helpful, but you have nowhere to go if that amp dies. If all your amps are [I]capable[/I] of 2 ohms but only [I]loaded [/I]to 4, if you lose a sub amp you can run your remaining good amp at 2 ohms and get through the night. Surely a much better result than being subless! Additionally, many amplifiers are just better behaved at 4 ohms so you’re actually getting less output and perhaps funny frequency response when you load them all the way down.

Aside from the damping factor issues, which are frankly kind of nit picky “last 5%” kind of optimizations, my concern with loading an amp down to 2 ohms a side is: what happens if it fails? I’m not saying that running it at maximum load will cause it to fail in and of itself, although for sure the extra thermal load is not helpful, but you have nowhere to go if that amp dies. If all your amps are [I]capable[/I] of 2 ohms but only [I]loaded [/I]to 4, if you lose a sub amp you can run your remaining good amp at 2 ohms and get through the night. Surely a much better result than being subless! Additionally, many amplifiers are just better behaved at 4 ohms so you’re actually getting less output and perhaps funny frequency response when you load them all the way down.[/QUOTE]

I agree completely and have been saving pennies for another sub amp. When I get one it will probably be placed on the other side so I can use a shorter cable. Hopefully things will act act right until I can afford another amp.

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